Many various forms of thermoplastic polymeric material are conventionally extruded into a wide variety of products including sheets, film, rods, tubes, strands, as well as many others. Basically, the extrusion process requires the melting of the thermoplastic material employed, and sufficient pressurization of the melted material to cause it to flow, at the desired rate, through an appropriate die to form the intended end product. Pressures in the range of 2000 to 5000 pounds per square inch (psi) in the melted material at the dieface typically are required. An important determinant of the quality of the end product is how well its dimensions conform to specification. Usually, end product quality decreases as the extrusion rate increases, and over the years, an ongoing problem has existed in the industry in striking an acceptable balance between an economical rate of extrusion and an acceptable quality of the extruded product.
For many years, conventional apparatus for extruding thermoplastic material generally as included a cylindrical barrel in which is rotatably disposed a conveying screw. The thermoplastic material is fed in a solid pellet or powder form into the barrel at one end and conveyed to the other end of the barrel by rotation of the screw. The heat required to transform the thermoplastic material from a solid at room temperature to a molten material at the desired extrudate temperature is derived from two sources. First, heating elements affixed to the extruder barrel generate heat which flows by conduction and convection into the thermoplastic material. Second, the motion of the thermoplastic material through the screw channel generates frictional heat. This friction heat is derived ultimately from the mechanical drive system of the extruding apparatus and is often referred to as mechanical heating.
The rate at which mechanical heating is generated in conventional extruding apparatus increases rapidly with increasing screw speed. In most extrusion processes, relatively high screw speeds, typically in excess of 100 rpm, are used to obtain economical production rates, and this results in frictionally generated heat being the dominant source of energy. Hence with increasing screw speed, and increasing extrusion rates, extrudate temperature rises often above a desirable level. Thus, in many extrusion processes, mechanical heat generation and extrudate temperature limits the speed and hence the productive capacity of the extruding apparatus. In order to operate at high screw speeds, many extruding apparatus perform cooling of the extruder barrel to remove the excess heat energy from the thermoplastic material. However, the heat removed by such cooling is waste heat thus making the process energy inefficient.
Conventionally, the extruder screw is constructed to define a helical channel of a decreasing cross-sectional areas whereby, in steady operation, the forces generated by the rotation of the extruder screw to force the thermoplastic material through the channel generate hydrostatic pressure in the thermoplastic material. This pressure causes the forward flow of the material to be diminished, an effect often attributed to and called "backflow", which has conventionally been considered advantageous in insuring complete melting and homogeneity of the melted thermoplastic material.
Conventional extruding apparatus is widely recognized to have several disadvantageous limitations. First and most notably, the rate of extrusion and the extrudate temperature uniformity and quality are inversely related to one another, which substantially limits the maximum extrusion rate at which an acceptably uniform product can be extruded and therefore correspondingly limits the productivity of such apparatus. The term "extrudate quality" generally refers to the uniformity of the temperature, pressure and composition of the extrudate at the dieface. If the extrudate is of low quality, temperature and pressure fluctuations are large and the flow of the thermoplastic material through the die will be irregular and unsteady and the quality of the product will be degraded. Frequently, in an attempt to achieve greater production rates with low quality extrudate, conventional extruding apparatus may be operated to produce a product having a greater than desirable average thickness in order to meet minimum thickness specifications. Secondly, as aforementioned, a substantial portion, usually 75-100% and often more, of the heat required in conventional extruding apparatus for melting the thermoplastic material is generated by mechanical heating. Hence, conventional extruding apparatus require powerful drive systems and operate with relatively low values of "power economy", defined as the ratio of the extrusion rate to the mechanical power expended. Typically, power economy is in the range of 5 to 10 pounds per horsepower-hour (lbs/HP-hr) in conventional extruding apparatus. Thirdly, as aforementioned, many conventional extruding apparatus are relatively energy inefficient as a consequence of their generation of waste excessive heat in the extrudate due to high screw speeds conventionally employed and the accompanying necessity of removing the excess heat energy by cooling.
In recent years, considerable interest has developed in the utilization of gear pumps, sometimes referred to as melt pumps, in combination with a conventional above-described screw extruder intermediate it and the die as a means of overcoming at least partially the described disadvantages of conventional extruders, a representative example of which is disclosed in U.S. Pat. No. 4,350,657. As will be understood, a gear pump is essentially a positive displacement device and therefore its volumetric output is a function of the pump speed and is substantially unaffected by pressure surges and fluctuations in the input flow from the extruder. Accordingly, the use of a gear pump will make more uniform the volumetric flow of melted thermoplastic material through the die of the extruding apparatus. However, experience has shown that passage through a gear pump will not significantly reduce temperature variations in the polymer melt. Hence a gear pump can only insure that the volumetric flow rate of the extrudate is uniform and if the extrudate has large temperature variations in it, these will pass through the pump and appear at the die, where they can cause irregular flow and degrade product quality. Experience has also shown that a combined extruder-gear pump system does not necessarily result in increased power economy, or reduced extrudate temperatures and that the overall energy efficiency of a pump-extruder system may actually in some cases be lower than that of the extruding apparatus itself. As the state of the art of combined systems of extruders and gear pumps has developed, it has remained conventional practice to operate the screw extruder under conditions comparable to conventional extruders alone with relatively high screw speeds and with mechanical heat generation predominating, as is represented in the aforementioned U.S. Pat. No. 4,350,657.